Conventional internal combustion engines typically include one or more camshafts in rotational communication with a crankshaft supported in a block, one or more intake and exhaust valves driven by the camshafts and supported in a cylinder head, and one or more pistons driven by the crankshaft and supported for reciprocal movement within cylinders of the block. The pistons and valves cooperate to regulate the flow and exchange of gases in and out of the cylinders of the block so as to effect a complete thermodynamic cycle in operation. To this end, a predetermined mixture of air and fuel is compressed by the pistons in the cylinders, is ignited and combusts, which thereby moves the piston within the cylinder to transfer energy to the crankshaft. The mixture of air and fuel can be delivered in a number of different ways, depending on the specific configuration of the engine.
Irrespective of the specific configuration of the engine, contemporary engine fuel systems typically include a pump adapted to pressurize fuel from a source (e.g., a fuel tank) and to direct pressurized fuel to one or more fuel injectors selectively driven by an electronic controller. Here, the fuel injectors atomize the pressurized fuel, which promotes a substantially homogenous mixture of fuel and air used to effect combustion in the cylinders of the engine.
In so-called “port fuel injection” (PFI) gasoline fuel systems, the fuel injectors are arranged up-stream of the intake valves of the cylinder head, are typically attached to an intake manifold, and are used to direct atomized fuel toward the intake valves which mixes with air traveling through the intake manifold and is subsequently drawn into the cylinders. In conventional PFI gasoline fuel systems, a relatively low fuel pressure of 4 bar (approximately 58 psi) is typically required at the fuel injectors. Because the pressure demand of PFI gasoline fuel systems is relatively low, the pump of a PFI gasoline fuel system is typically driven with an electric motor.
In order to increase the efficiency and fuel economy of conventional internal combustion engines, the current trend in the art involves so-called “direct fuel injection” (DFI) fuel system technology, in which the fuel injectors introduce atomized fuel directly into the cylinder of the block (rather than up-stream of the intake valves) so as to effect improved control and timing of the thermodynamic cycle of the engine. To this end, modern gasoline DFI fuel systems operate at relatively high fuel pressures, for example 500 bar or higher (approximately 7300 psi). Because the pressure demand of DFI fuel systems is relatively high, a high-pressure fuel pump assembly which is mechanically driven by a rotational movement of a prime mover of the engine (e.g., one of the camshafts) is typically employed. Thus, in many embodiments, the same camshaft used to regulate valves in the cylinder head is also used to drive the high-pressure fuel pump assembly in DFI fuel systems. To this end, one of the camshafts typically includes an additional lobe that cooperates with a tappet supported in a housing to translate rotational movement of the camshaft lobe into linear movement of the high-pressure fuel pump assembly.
The high-pressure fuel pump assembly is typically removably attached to the housing with fasteners. The housing of the high-pressure fuel pump assembly may be formed as a discrete component, or may be realized as a part of the cylinder head, and includes a tappet cylinder in which the tappet is supported for reciprocating movement.
The tappet typically includes a bearing which engages the lobe of the camshaft, and a body which supports the bearing and is disposed in force-translating relationship with the high-pressure fuel pump assembly. Here, the high-pressure fuel pump assembly typically includes a spring-loaded piston which is pre-loaded against the tappet body when the high-pressure fuel pump assembly is attached to the housing. Thus, rotational movement of the lobe of the camshaft moves the tappet along the tappet cylinder of the housing which, in turn, translates force to the piston of the high-pressure fuel pump assembly to displace and pressurize fuel. As the lobe of the camshaft continues to rotate, potential energy stored in the spring-loaded piston of the high-pressure fuel pump assembly urges the tappet back down the tappet cylinder such that engagement is maintained between the bearing of the tappet and the lobe of the camshaft.
During engine operation, and particularly at high engine rotational speeds, close tolerance must be maintained between the lobe of the camshaft, the tappet, and the piston of the high-pressure fuel pump assembly. Excessive tolerance may result in poor performance as well as increased wear, which leads to significantly decreased component life. Thus, it will be appreciated that it is important to maintain predetermined tolerances between the lobe of the camshaft, the tappet, and the piston of the high-pressure fuel pump assembly under varying engine operating conditions, such as engine rotational speed or operating temperature.
Each of the components of an internal combustion engine high-pressure fuel system of the type described above must cooperate to effectively translate movement from the lobe of the camshaft so as to operate the high-pressure fuel pump assembly at a variety of engine rotational speeds and operating temperatures and, at the same time, maintain correct tolerances so as to ensure proper performance. In addition, each of the components must be designed not only to facilitate improved performance and efficiency, but also so as to reduce the cost and complexity of manufacturing and assembling the fuel system, as well as reduce wear in operation. While internal combustion engine high-pressure fuel systems known in the related art have generally performed well for their intended purpose, there remains a need in the art for a high-pressure fuel system that has superior operational characteristics, and, at the same time, reduces the cost and complexity of manufacturing the components of the fuel system.